National Seasonal Rainfall Outlook: probabilities for August to October 2003,
issued 16th July 2003

Rainfall odds mostly neutral for late winter/mid-spring

The Bureau's seasonal rainfall outlook
for the late winter to mid-spring period, shows near 50:50 chances
across most of the country with no big swings towards wetter or drier
conditions across any State or Territory.

For the August to October period, the chances of above median rainfall are
between 40 and 60% across most of the country (see map). So with
climate patterns like the current, about 5 seasons out of 10 are
expected to be wetter than average, with about 5 out of 10 being drier.

It should also be noted that August and September are in the heart of the
dry season across northern Australia and heavy rain at that time
of year is uncommon.

The Bureau's August to October rainfall outlooks have moderate reliability
over much of Queensland and the north and east of the NT, but mainly low reliability
elsewhere (see background information).

The Outlook probabilities are based on recent
Indian and Pacific Ocean temperatures.
The tropical Indian Ocean is warmer than average, whilst
the Pacific is showing a mixed pattern of warmer and cooler
than average. For more detail see the El Niño Wrap-Up.
The excess warmth in the Indian Ocean has had the greater
influence on the overall pattern of probabilities.

June's value of the Southern Oscillation Index
(SOI) was 12,
five points below the May value of 7.
The approximate SOI for the 30 days
ending 13th July was 3.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

The following climate meteorologists in the National Climate Centre
can be contacted about this outlook:
Grant Beard on (03) 9669 4527,
Janita Pahalad on (03) 9669 4859,
David Jones on (03) 9669 4085,
Rob Nash on (03) 9669 4288.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Reliability:
Strong reliability means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median).
Low reliability means the historical relationship is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).